How to Teach a Simple Liquid New Tricks -
the Curious Behavior of Liquids Under Nanoconfinement

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That solid materials show new phenomena at the nanoscale is
well known, but the behavior of nanoscale liquids is less well understood.
Liquids confined to the nanoscale are of great importance in biology, where the
average spacing between macromolecules in a cell is of order nanometers, and in
nanotribology, where friction is due to the interaction between surfaces
entrapping nanoscale liquid films. Atomic force microscopy (AFM) and surface
force apparatus (SFA) have been used to confine liquids to nanoscale spaces and
to measure their structural and mechanical properties. Unfortunately, these
studies have produced contradictory results, ranging from a dramatic spontaneous
solidification to no change at all compared to bulk behavior. While the
molecular short-range order (layering) along flat surfaces is well established,
the mechanical response of the liquid is still highly controversial with
sometimes diametrically opposed results reported for the same liquid under
identical conditions. However, it seems clear that drastic changes in structure
and mechanical behavior do not emerge until the liquid is squeezed down to less
than 5-6 molecular layers. It also seems clear that some of the controversy can
be resolved if we realize that the mechanical response of the liquid depends on
its dynamic state and the used measurement technique (area, stiffness,
frequency, amplitude). In this talk, I will review the history of the field, and
then discuss some of the most recent results, including AFM results on water and
the silicone oils OMCTS and TEHOS obtained in our nanomechanics lab at Wayne
State University in Detroit, USA. I will argue that nanoconfined liquids behave
in ways that do not have a counterpart in bulk liquids, even considering the
complicated behavior of some non-Newtonian fluids.